If one cuts an onion with a meaty knife on a meaty cutting board, can eat that onion with tuna fish?
It is permitted to eat the tuna with the onion.
The Taz (Yoreh De’ah 95:3) explainsthat the prohibition of eating a mixture of meat and fish applies only to eating actual meat and fish together. The taste of meat alone (ta’am), which is emitted by a pot or plate into fish, does not render it as forbidden food, and vice versa.
This is why we may eat fish with clean meat utensils. Likewise, after eating hot fish with a fork, it is sufficient to wash the fork, before using it for meat. We are not concerned for the belios in the fork.